This invention relates generally to densitometers for measuring optical density. In particular, the invention relates to optical density measurement of ink-covered or toner-covered surfaces produced by apparatus such as printers, and to photographically printed areas.
In electrostatographic apparatus such as copiers and printers, automatic adjustment of process control parameters is used to produce images having well regulated darkness or optical density. Copier and printer process control strategies typically involve measuring the transmissive or reflective optical density of a toner image on an exposed and developed area (called a xe2x80x9ctest patchxe2x80x9d) of an image receiver. Optical density has the advantage, compared to transmittance or reflectance measures, of matching more closely to human visual perception. A further advantage, especially for transmission density, is that density is approximately proportional to the thickness of the marking material layer, over a substantial range.
Typically, toned process control test patches are formed on the photoconductor in interframe regions of the photoconductor, i.e., between image frame areas. An xe2x80x9con-boardxe2x80x9d densitometer measures the test patch density, either on the photoconductor or after transfer of the patches to another support member. From these measurements, the machine microprocessor can determine adjustments to the known operating process control parameters such as primary charger setpoint, exposure setpoint, toner concentration, and development bias.
A transmission type of densitometer is particularly well suited to transmissive supports. In this type, a light source projects light, visible or infrared, through an object onto a photodetector such as a photodiode. In a copier/printer, the photoconductor passes between the light source and the photodetector. When the photoconductor has toner on the surface, the amount of light reaching the photodetector is decreased, causing the output of the densitometer to change. Based on this output, the amount of toner applied to the photoconductor can be varied as required in order to obtain consistent image quality. Another type of densitometer such as described in U.S. Pat. No. 4,553,033 to Hubble, III et al uses reflected light flux rather than transmitted light flux to determine density, and is particularly suited to opaque reflective supports.
One well-known approach to converting to a density measure uses an analog logarithmic amplifier, as suggested by the mathematical logarithm function in the definition of optical density:
D=xe2x88x92log10(T)xe2x80x83xe2x80x83Equation (1)
where D is optical density, and T is transmittance or reflectance (for transmission density or reflection density, respectively). The subscript xe2x80x9c10xe2x80x9d indicates that the logarithm is to the base 10. Since T must be between 0 and 1, the logarithm of T is negative, and the minus sign (xe2x88x92) in equation (1) provides positive values for density, D.
The following U.S. Patents, for example, all teach the use of an analog logarithmic amplifier in a densitometer: U.S. Pat. No. 3,918,815 to Gadbois, U.S. Pat. No. 5,148,217 to Almeter et al, U.S. Pat. No. 5,173,750 to Laukaitis, and U.S. Pat. No. 5,903,800 to Stern et al. The high cost of precision analog logarithmic amplifiers does not seriously deter their use in expensive laboratory instruments. However, the high cost of analog logarithmic amplifiers has been an obstacle to the wide use of densitometers as built-in components within moderately priced copiers, printers, and other products.
Digital approaches to densitometer design have been developed, as digital electronics improve in performance and decline in price, relative to analog logarithmic amplifiers. One digital approach in the prior art is to obtain a photodetector voltage signal representing intensity of transmitted or reflected light and convert this analog signal to digital form. The digital value is then used to enter a stored lookup table (LUT) of and density values. The digital density value corresponding to the digital intensity value is read from the LUT. To cover a reasonably large range of density with the required resolution, an amplifier with selectable gain has been used.
U.S. Pat. No. 5,117,119 to Schubert et al discloses an automatic gain selection, i.e., an xe2x80x9cauto-rangingxe2x80x9d electronic circuit, along with a second LUT, to obtain high accuracy and resolution over an increased range of large densities. The first (or xe2x80x9cbasexe2x80x9d) LUT contains density values corresponding to an analog-to-digital (A/D) converter output for the lowest gain. The second (or xe2x80x9crangexe2x80x9d) LUT is much smaller than the first LUT and contains the relative density corresponding to each available gain. It provides the density increment associated with the gain selected. The two LUT outputs are summed to obtain the actual density measurement. LUT approaches are also disclosed by Rushing et al in U.S. Pat. Nos. 6,222,176 and 6,225,618, and by Rushing in U.S. Pat. No. 6,331,832.
Prior art digital densitometers, such as those in the aforementioned disclosures, typically have an analog amplifier stage near the photodetector end of the circuit, before converting the measurement signal to digital form. While the signal is in analog form, it is especially vulnerable to corruption by electrical noise and small voltage offsets.
Moreover, the analog amplifiers in prior art densitometers have a selectable gain, requiring the switching of the analog measurement signal. Even small noise levels and error introduced in switching can have a relatively large effect on a small measurement signal. For example, integrated circuit analog switches typically have an xe2x80x9cONxe2x80x9d resistance of several 10""s to a few 100""s of ohms, which varies with the voltage being switched and other factors. This resistance alters the amplifier gain, introducing a variable error, which is difficult to compensate. Unwanted switching transients during gain changes compound the problem.
Another digital approach to digital densitometry is suggested, without design specifics, by Edwards in the November, 1996 issue of xe2x80x9cNuts and Voltsxe2x80x9d magazine. Edwards suggests using a light-to-frequency (L-to-F) converter integrated circuit as a photodetector in a densitometer. In U.S. Pat. No. 6,188,471, Jung et al disclose the use of such L-to-F converters for the measurement of optical properties such as color, gloss, and translucence. Jung et al do not address the logarithmic conversion underlying optical density measurement.
In a L-to-F converter, the output frequency is proportional to the incident light intensity. Reflected or transmitted light incident on the converter can be measured in intensity by one of two methods, as suggested in the xe2x80x9cTSL230 Evaluation Module Users Guide,xe2x80x9d by Texas Instruments, Inc. One method is to count the converter output pulses during a fixed time period, yielding a frequency count Optical density can then be determined from the known relationship of intensity, frequency and optical density. The other method is to measure the period or pulse width by counting clock pulses during a single output pulse from the converter, yielding a period count. Optical density can then be determined from the known relationship of intensity, converter output period and optical density.
In the frequency count method, updated density measurements are obtained only as often as the fixed counting time period. This counting time must be long enough to provide a large frequency count to obtain good density resolution, even for low frequencies (high optical density).
In the period count method, updated density measurements are obtained with every measured period For very high incident light intensities (low optical density) the frequency is very high, and the period so short that the count of clock pulses is small, and the conversion to density has insufficient resolution. For very low incident light intensities (high optical density), the frequency is very low, and the period so long that the count of clock pulses may cover a very large range. Such a large range is unwieldy, and if used to address a LUT, the LUT must be very large.
For frequency measurement over a wide range, the frequency count and period count methods have been combined, with automatic switching between the two, to get optimum resolution for any input frequency. This approach is described by Horowitz and Hill in xe2x80x9cThe Art or Electronics,xe2x80x9d as having been used commercially in the Hewlett Packard Co. model 5315A.
U.S. Pat. No. 6,144,024, to Rushing, combines the frequency count method and the period count method, to achieve wider density measurement range. Dual counters and dual density LUTs are used For very high densities (low light intensity at the sensor, and low L-to-F output frequency), the large period counts require a large LUT, i.e., a LUT with many entries.
Since logarithmic conversion is at the heart of densitometry, logarithmic converters in other contexts may bear on densitometer applications. U.S. Pat. No. 5,341,089 to Heep discloses a digital circuit to convert an analog voltage input to decibel (dB) units. The dB output is defined as 10 times the logarithm (base 10) of the ratio of the power of the input signal relative to a reference power level. This logarithmic conversion is of the same general type as used within densitometers, according to equation (1). The Heep disclosure has no selectable gain and no auto-ranging. Large inputs must first be scaled down by a manually adjusted voltage divider to obtain an input within the operating range, and an output from a second LUT is added to compensate for the scaling down. Interpolation between LUT values is applied to obtain the desired accuracy, adding complexity to the circuit and lengthening the time required obtaining a measurement update.
One object of the present digital densitometer invention is to obtain a reasonably rapid update rate and good density resolution over a wide density range, with minimum circuit complexity. The present densitometer invention uses only a single lookup table, which can be relatively small, and is less complex than digital densitometer circuits in the prior art.
Another object of the present invention is to process the measurement signals more completely in digital form, to obtain the superior noise immunity of digital signal processing. The analog light intensity signal is immediately converted to a digital form, i.e., a variable-frequency square wave oscillating between logic xe2x80x9c0xe2x80x9d and logic xe2x80x9c1xe2x80x9d, within the light-to-frequency converter portion of the circuit. In subsequent stages, the circuit processes the signal digitally. No switching of analog signals is involved.
It is still another object of the present invention to provide a digital logarithmic converter circuit, applicable not only within densitometers, but also in other applications where there is need to convert an input to an output proportional to the logarithm of the input. A digital logarithmic converter could be applied in a variety of fields. In photography the xe2x80x9cf-stopxe2x80x9d measure is proportional to the logarithm of aperture area In chemistry the xe2x80x9cpHxe2x80x9d acid-base measure is proportional to the logarithm of ion concentration. In acoustics, the xe2x80x9cdBxe2x80x9d sound intensity measure is proportional to the logarithm of the sound power. In electronic circuits, xe2x80x9cdB gainxe2x80x9d is proportional to the logarithm of the voltage gain. In general, any signal varying over a wide dynamic range may be evaluated more readily without switching gain or scale by first passing the signal through a logarithmic converter.
To obtain the aforesaid objects, a densitometer using a light-to-frequency (L-to-F) converter is disclosed. Transmitted or reflected light is converted to an oscillating signal with frequency characteristic of the light intensity incident on the converter. With a programmable L-to-F converter, digital controls for sensitivity and/or divide-by ratio are input to the L-to-F converter to obtain an output frequency within a relatively narrow range. A period counter measures the period of the converter output. A LUT address is formed at least partially from the period count. The LUT output is proportional to the logarithm of the period count, with a positive or negative offset. The LUT output offset may be removed by shifting the output value according to the programmed sensitivity and divide-by ratio, to obtain a scaled density value, or at least the low-order digits of a scaled density value.
With a non-programmable L-to-F converter, large period counts may be numerically divided such that the quotient, i.e., the divided period count, is within a relatively narrow range. A LUT is addressed according to the period count, or divided period count. The number of entries, i.e., number of addresses, in the LUT is small, owing to the period count, or divided period count, being within a relatively narrow range. The LUT output is proportional to the logarithm of the period count, or divided period count, with a positive or negative offset. The LUT output offset may be shifted by addition or subtraction, and thereby removed, according to the divisor used to obtain the divided period count, to obtain a scaled density value.
To extend the measurement range to higher light intensities incident on the L-to-F converter (for lower density test samples), a frequency count is used to address the same LUT addressed by the period count when measuring higher densities. One aspect of the invention uses a relatively high intensity light emitter, and switches automatically to the frequency count addressing when the period count is too small for acceptable density resolution. The LUT output is processed according to whether the LUT address was derived from period count or frequency count, and according to the divisor used. Only a single LUT, relatively small in size, is addressed according to either period count or frequency count, enabling a large density measurement range.
The L-to-F converter may be adapted to process analog voltage inputs rather than light inputs. The logarithmic conversion then becomes applicable to a variety of fields besides densitometry.
The invention and its various advantages will become more apparent to those skilled in the art from the ensuing detailed description of the preferred embodiments, reference being made to the accompanying drawings.